The invention relates to a two-stage brake valve.
A two-stage brake valve has become known from DE-A-1 680 211 in which the large effective cross section is formed by a pressure piston, which is arranged and sealed in a blind bore in the valve housing and which can be moved in the blind bore by a piston rod. The first pressure chamber corresponding to the larger effective cross section is located ahead of the end face of the pressure piston. The pressure piston contains an axial bore open to its end face, that engages a shank attached to the valve housing and that can move within the bore. A second pressure chamber is formed within the pressure piston with a smaller effective cross section. The shank is provided with a central bore through which the second pressure chamber is connected to the brake line. The first pressure chamber is connected to a reservoir through a check valve arranged in a return line (relief channel). When a pre-set pressure is exceeded in the first pressure chamber, the check valve opens. A valve arrangement is located between the first pressure chamber and the second pressure chamber, which permits an interchange of fluid between the pressure chambers and seals these against each other when a pre-set pressure is exceeded. During a first phase of movement of the pressure piston the brake chamber of the brake is filled until the pressure in the first pressure chamber rises to a point at which the check valve opens. Upon further actuation of the pressure piston the pressure in the second pressure chamber rises further and leads to closing of the valve arrangement between the pressure chambers, so that now only the smaller effective cross section of the shank is effective and a high braking pressure can be applied with relatively lower actuating force.
U.S. Pat. No. 4,963,692, issued Oct. 16, 1990 to Halabiya, describes a brake control valve that is designed similar to the one described above, in which, however, the shank does not engage the bore in the pressure piston when the pressure piston is in its rest position. Such an engagement during which the chamber is sealed occurs only after a pre-set path of movement of the pressure piston during which time the brake chamber is filled up to the point of contact of the brake linings. As soon as the shank engages the bore of the pressure piston the first pressure chamber is separated from the second pressure chamber and the smaller effective cross section becomes effective, which builds up the actual brake pressure. A valve arrangement between the two pressure chambers is not required with this brake control valve. The first pressure chamber is connected to a reservoir during the fill phase through an overpressure relief valve arranged in a relief channel, and through a direct relief channel during the pressure phase.
In these and similar two-stage valves there is the problem that with high forces acting on the pressure piston, excessively high operating pressures can be built up in the brake system or in another attached system. These pressures can lead to damage of the brake or another system.
Commonly, protection against excessive pressure can be attained by a blocking valve or a relief valve. Such a blocking valve blocks the flow of hydraulic fluid to the system after a pre-set pressure is reached. However, in brake applications this is not acceptable. For example, in brakes, at high pressures sudden leakages and other damage may not be noticed immediately. If a relief valve is used, it releases the hydraulic fluid from the system when the pre-set pressure is exceeded, until the operating pressure is reduced below the pre-set pressure. In brakes this is also unacceptable since the brake pedal would then sink to the floor.